On Tue, May 09, 2000 at 06:46:44PM +0200, Nicolas Huillard wrote:
> I'm finishing reading Dr Le Blanc's presentation of Coda
> (http://www.mcc.ac.uk/~zlsiial/coda/text47.html) and also wonders
> if... (quote :) "Finally, I wonder whether Coda's success as
> high-availability software is impaired by the dependence of a cell on
> a single System Control Machine." I am really concerned about this
> issue, because my goal is to build High Availability...
> Does anyone have an idea ?

The coda clients and servers do not rely on the existence of an SCM. The
only reason that machine is special is because of the way we propagate
updates to the user and volume databases. They are read/write on the
SCM, and the other servers check for updated files on the SCM once every
30 seconds.

If you lose the SCM, all servers can still rely on their local copy. But
in order to add a volume or user, you need to restore the old SCM. Or
designate one of the other servers (all of them should have all of the
required data) as the replacement SCM. To do this is is necessary to
update the /vice/db/scm file on all servers, and restart the
updateclnt/updatesrv processes. Not as good as a quorum based replicated
database server, but a lot simpler for the common case.

Jan